A typical radio transmitter uses a radio frequency (RF) power amplifier to amplify outbound signals for transmission by an antenna. A linear power amplifier receives a signal at an input power and produces the signal at an output power, which is the input power amplified by a gain. The linear power amplifier is driven by a direct current (DC) input voltage, provided for example by a battery in the transmitter, and the efficiency of the power amplifier is given by the ratio of the output power to the DC input power. RF power amplifiers are generally designed to provide maximum efficiency at the maximal output power. When the power amplifier produces an output power that is less than the maximal output power, the efficiency of the power amplifier may be significantly reduced.
There are several situations where the output power of an RF transmitter needs to be less than the maximal output power (xe2x80x9cpower controlxe2x80x9d). For example, in amplitude modification (AM) radio transmission, the instantaneous output power is set according to the instantaneous strength of the modulating audio to be transmitted. In the case of battery-operated portable handsets such as cellular telephones and pagers, the power amplifier is one of the main current consumption elements, and power control is used in order to prolong the lifetime of the battery. Power control is also used to reduce interference between different calls in the same vicinity and to reduce the radiation emitted by the handset. In fact, modern cellular systems allow handset transmission at low power for long periods of time.
However, since the efficiency of the power amplifier is significantly reduced when the power amplifier produces an output power that is less than the maximal output power, more battery power at lower output power is actually consumed. It would therefore be beneficial to improve the efficiency of the RF power amplifier at lower output powers without reducing the efficiency at the maximal output power.